The present invention relates to methods and structures for uprating or improving electric machines and, more particularly, to reducing core-end losses of dynamoelectric machines using lower core loss iron silicon.
Generator power output is limited by significant heat buildup on the extreme ends of the generator field due to high eddy current losses. The flux associated with the field induces eddy currents in the stator teeth, and penetrates several inches into the stator core. The heating caused by the eddy currents is a limiting factor in the design of large electric machines. In addition, the axial fluxes add onto the normal radial fluxes in the end packages, increasing the level of saturation in the end teeth and yoke. The effect of reducing radial flux entering the first and second packages is to lower the level of saturation in these packages, enabling the axial flux to turn radially closer to the core end. The resultant effect is to lower the axial flux densities, and therefore the iron temperatures.
Many ways are being used to reduce the electromagnetic flux normal to the stator core-end that causes eddy current losses and heating. Increasing the air-gap length toward the end of the stator core reduces such core end flux. Core-end stepping is one of the commonly used techniques to modify the air-gap length in the core-ends (see FIG. 1). Reducing the relative axial length of the rotor with respect to the stator is another way to reduce the core end fringing flux. The amount of stepping is limited, however, by the need to support the lamination “teeth” from flexing. Loose lamination teeth will flutter and break off causing machine failure. These fixes also increase the excitation needs contributing to lower efficiencies and potentially a bigger thermal challenge.